ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
    ListNode *h = nullptr, *t = nullptr;
    int carry = 0;
    while(l1 || l2)
    {
        int n1 = l1 ? l1->val : 0;
        int n2 = l2 ? l2->val : 0;
        int sum = n1 + n2 + carry;
        carry = sum / 10;
        if(!h)
            h = t = new ListNode(sum % 10);
        else
        {
            t->next = new ListNode(sum % 10);
            t = t->next;
        }
        if(l1) l1 = l1->next;
        if(l2) l2 = l2->next;
    }
    if(carry) t->next = new ListNode(carry);
    return h;
}